Machines used for manufacturing and testing semiconductor components often require that objects be precisely positioned therein. Thus, the need can arise, for instance, for highly accurate positioning of wafers underneath a tool of an exposure or inspection unit. The wafer resides on a stage that is movable in six degrees of freedom and that is moved by a corresponding drive. Accordingly, the stage functions as an object whose position is to be measured with a high degree of accuracy. Positioning this stage over the drive and a corresponding control unit requires highly accurate position-measuring devices to generate positional signals indicative of the spatial position of the stage.
In such machines, interferometers or grating-based optical position-measuring devices are used as highly accurate position-measuring devices. German Patent Application No. 10 2012 201 393.3, U.S. patent application Ser. No. 13/757,533, and U.S. Provisional Application Ser. No. 61/593,582, each of which is expressly incorporated herein in its entirety by reference thereto, describe a measuring system for such a use that combines different position-measuring devices. Thus, a multi-axis interferometer is used for measuring positions along a first, long principal axis of motion of the object and for recording rotational movements about other axes. Provided on the object in this case is a measuring reflector, to which measurement beams of the multi-axis interferometer are incident. Interferential position-measuring devices are provided for measuring positions along a second principal axis of motion and along a third axis. They include, for example, measuring standards provided on the object in the form of incident-light diffraction gratings, as well as reflectors that are fixed relative thereto. Other diffraction gratings are provided on the reflectors, it being possible for the reflectors to be arranged as transmitted-light grating-mirror units, for example. During assembly and use, the measuring reflectors, measuring standards, and reflectors used in the various position-measuring devices are subject to mechanical, as well as thermal stresses and may also undergo slow deformation in the process. Such deformations of these elements result in measurement errors when determining positions.
To compensate for these measurement errors, what are generally referred to as self-calibration methods are used, for example, for measuring and correcting the active deformations of the particular elements, such as, for example, the measuring standards, respectively reflectors, during operation or during special calibration cycles of the particular machine. These types of self-calibration methods generally require the measuring standard or the reflector to be scanned along the particular extension direction by two or more scanning, e.g., optical, units, and the positional signals to be generated by both optical units. The positional signals of the two optical units are then differentially transmitted, so that, from the resulting differential signal, the specific existing, deformation-induced error of the particular measuring standard or reflector can be calculated and subsequently compensated for, e.g., in a conventional manner. With regard to such self-calibration methods, reference is made, for example, to the publication, “Exact Wavefront Reconstruction from Two Lateral Shearing Interferograms,” C. Elster, I. Weingärtner in Vol. 16, No. 9, September 1999, J. Opt. Soc. Am. A, 2281-2285, which is expressly incorporated herein in its entirety by reference thereto.
This means that, per measuring direction, such a self-calibration method requires two optical units for scanning and positional signal generation. This constitutes substantial additional expenditures and, consequently, an increase in the unit volume of the entire system.
Regardless of the described self-calibration problems, it may also be necessary or advantageous for other reasons to redundantly measure positions along one direction of movement in certain measuring sequences or machine states. Thus, for example, at certain positions, the requisite number of what are generally referred to as Abbe arms may be reduced or the accuracy enhanced.